As near-eye displays, such as virtual reality goggles, and heads-up displays become more prevalent, optical issues such as the pupil size of the optical apparatus (sometimes referred to as the exit pupil size) have become more important. In such applications, a bigger pupil size is preferred, however, a trade-off exists between field of view (FOV), pupil size, the f-number of the optics, and a display panel size that is to be seen through the optics of the system. For a given display panel size and f-number, the FOV decreases as pupil size increases. In near-eye displays in particular, the size, weight, and cost of the display panel size mean that smaller display panels are more desirable, which tends to mean that the pupil size is also smaller.
Optical systems used to create an image suitable for viewing with the eye are designed to create a virtual image. This virtual image can only be seen in its entirety when the observer's eye pupil is inside a viewing area known as the exit pupil of this optical system. It is more comfortable and ergonomic for the viewer of the virtual image if this exit pupil is large compared to the eye's pupil since small motions of the head or eye do not affect the users view of the virtual image. A large eyebox is normally difficult to create, as it requires lenses and mirrors that are larger than desired especially for wearable near eye displays. The development of waveguide pupil expanders has offered a potential solution to this problem by allowing for the optical system exit pupil to start small at the input to the waveguide and then expand becoming large at a designated exit area.
An exit pupil of the optical system designed to create a virtual image is placed at the entrance to a waveguide specifically designed as a pupil expander. The waveguide pupil expander works by coupling in this pupil light with a diffraction grating which changes the input light ray angles so that they remain trapped inside and travel along the waveguide by the phenomena of total internal reflection. When the light reaches the exit port area of the waveguide a second diffraction grating slowly releases the trapped rays allowing them to escape the over a larger area. The difference between the input area where the original optical system pupil was coupled into the waveguide and the expanded pupil over the area where the light was allowed to leak out of the waveguide is the degree of pupil expansion. In any method of pupil expansion, the angle of the light rays must be preserved from the input to the output as any ray angle deviations will lead to image artifacts such as distortions or even multiple ghost images if the ray angles flip direction.
Waveguides can be designed to expand the pupils in one or two dimensions depending on the design of the diffraction gratings and internal structure. However, pupil expansion by allowing light to leak out of the exit port of the pupil expander waveguide creates several image artifacts which are undesirable. As the light rays are slowly diffracted out the energy in those rays slowly decreases. This leads to a brightness decrease in the virtual image brightness depending on the position of the eye in the expanded pupil. Another problem which is related to this is the shifting in color of a white image due to different amounts of blue, red and green light diffracted out of the waveguide at different rates. These are just a few of the artifacts possible but all are related and get more severe as the pupil is expanded to a greater extent.